Middle-aged adults who live alone are twice as likely to develop dementia or Alzheimer's disease later in life compared to those who are married or live with a partner. And the risk is three times higher among those who are divorced or widowed, according to a new study by Swedish and Finnish researchers.

The study included 2,000 men and women in Finland who were initially surveyed when they were 50 years old and again 21 years later.

In addition to looking at the association between marital status and dementia, researchers also examined the link between living alone and being a carrier of apolipoprotien E e4 gene variant, a known risk factor for Alzheimer's disease.